Capillary electrophoresis is a powerful separation technique making it possible to detect the presence of substances in detection zones of very small volume. This electrophoresis technique is particularly advantageous for application in the medical and biological field, with electrophoretic migration taking place either in a buffer or else on a gel.
Several detection modes (UV spectrometry, amperometry, . . .) may be used, but detection by fluorescence has been found to be particularly sensitive. See, for example, U.S. Pat. No. 4,675,300 issued Jun. 23, 1987, the disclosure of which is incorporated herein as background material by this reference. Fluorescence is the property possessed by certain substances whereby, when they are excited by light at a certain wavelength, they emit radiation at a wavelength longer than that of the incident rays.
The sensitivity of fluorescence detection comes from the fact that background emission (when not fluorescing) is practically nil so that large changes in emission are observed. This is in contrast to the conventional absorption phenomenon where the difference between the incident radiation and the emitted radiation is very small. In addition, selectivity is high due to the fact that two wavelengths are chosen (excitation energy and emission energy): this very precise choice makes very high selectivity possible.
In conventional fluorescence detectors, the light emitted by fluorescence is generally picked up either in the plane of the incident rays, or else (as taught by U.S. Pat. No. 4,675,300) perpendicularly to the plane of the incident rays.
Because of the tiny quantities of substances present, the amount of light emitted is low and it is necessary to provide optical devices either for separating the incident light from the emitted light, or else for reflecting the emitted light in a particular direction.
Such additional devices make it necessary to observe the light at a certain distance from the capillary, thus giving rise to a loss of sensitivity.